Tissue engineering holds great promise in creating functional tissues that can replace diseased or lost tissues of human beings. Recently, consensus has been reached that three-dimensional (3D) tissue culture is superior to traditional two-dimensional (2D) cell culture in recapitulating the in vivo cell microenvironments and tissue structures. It has been found that many engineered tissues are functional only when they are developed in 3D systems. Despite the recognized importance of 3D tissue engineering and the tremendous efforts that have been made, the progress of developing large and viable 3D tissues of clinically relevant sizes has been limited. One major challenge in creating such tissue products is insufficient mass transfer in the interior region of large and a vascular constructs. Although mass transfer in large constructs prepared from preformed porous scaffolds can be enhanced in vitro through perfusion culture, insufficient mass transfer remains a problem after these constructs are implanted in vivo. On the other hand, some in situ forming hydrogels allow encapsulated endothelial cells to form capillary networks that undergo an anastomosis with the host vasculature after implantation, but hydrogel constructs without large pores cannot be perfusion-cultured so that their size is limited. Lack of methods to create large perfusable hydrogel constructs supporting in vitro endothelial capillary morphogenesis for prevascularization limits our ability to address the problem of insufficient mass transfer in 3D tissue engineering. The objective of this R21 application is to use a modular assembly approach to develop large, porous hydrogel constructs containing endothelial capillary networks and to examine postimplantation survival of the cells in these constructs. The central hypothesis of this work is that fibrin microgels having well-controlled morphology and laden with endothelial cells and other cells of interest can be modularly assembled into large, porous constructs in situ through a judiciously selected chemical reaction occurring under physiologically permissive conditions and such assembled constructs can be perfusion- cultured in vitro and develop into prevascularized, porous constructs that support high postimplantation cell survival. The Specific Aims of this project are: (1) design, fabrication, and characterization of modularly assembled large porous fibrin hydrogels laden with human umbilical vein endothelial cells (HUVECs) and hMSCs; (2) in vitro culture of large porous cell-laden constructs under perfusion and characterization of capillary morphogenesis and cell viability; (3) implantation of prevascularized porous constructs and characterization of in vivo function of the capillary networks and postimplantation survival of transplanted cells. The method proposed in this application will provide a platform to create centimeter-sized porous constructs containing capillary networks that undergo anastomosis with the host vasculature after implantation and allow interstitial flow of body fluids. Successful accomplishment of this project will address the problem of insufficient mass transfer that hampers creation of functional 3D tissue products of clinically relevant sizes. (End of Abstract)